GRBs and Magnetic Fields

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GRBs and Magnetic Fields

• Shiho Kobayashi (????)
• Liverpool John Moores University

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How to constraint the magnetization (and
structure) of GRB ejecta

• Early afterglow
• RS and FS modeling
• (Synchrotron and IC)
• Early polarimetric measurements
• Ejecta structure Optical/X-ray polarimetry

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the synchrotron shock model is successful, but
there are some open questions

• Origin of Magnetic fields
• prompt 106G, afterglow 1G
• How to accelerate and collimate jets?
• gammagt100, E1052 ergs
• How to produce prompt gamma-rays
• internal shocks/efficiency issue
• Lack of optical flash in most events
• magnetic pressure?, SSC?

(MedvedevLoeb1999 Nishikawa et al. 2008
Gruzinov 2001 Milosavljevic et al. 2006)
(Usov 1992 MeszarosRees1997Lyutikov Blandford
2002 Drenkhahn Spruit 2002..)
(Kumar1999 Beloborodov2000 SKSari2000Zhang et
a. 2006 Nousek et al. 2006)
(Akerlof et al. 2000 Roming et al. 2006..)
Magnetized jet model might solve these.
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The Standard model
Energy transfer
relativistic outflow (ejecta from central engine)
Blastwave (FS ambient medium)
Emission from Ejecta Prompt gamma-rays Optical
Flashes Radio Flares? X-ray Flares?
Emission from Blastwave Afterglows
(X/Opt/Radio) Insensitive to the properties of
the original ejecta
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Method 1 RS and FS modeling
Forward Shock
Reverse Shock
ejecta
At the deceleration time (onset of afterglow)
The deceleration happens when a significant
fraction of the ejecta energy is given to the
forward shock region.

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• RS region energy per particle smaller by
• cooling frequencies comparable
• the number of electrons is proportional to mass
  

SKZhang2003
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If RS region has higher magnetization or
magnetic energy density expressed as a fraction
of the equipartion vale
Zhang,SKMeszaros 2003
Using these relations and theoretical decay
indexes of FS and RS emission, we can model
early afterglow
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GRB 061126
Optical Light curve
Gomboc, SK, Guidorzi, Melandri, Mangano et al.
2008
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RS and FS modeling RS region higher
magnetization, but still a baryonic jet
Gomboc et al. 2008
PanaitescuKumar2002
GRB 990123, GRB 021211 Zhang et al. 2003
KumarPanaitescu2003 Fan et al. 2005
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Method 2 Sync and SSC emission
Wus talk yesterday Synchrotron self-inverse
Compton radiation from RS the relative strength
Syn, 1st IC and 2nd IC components depends on the
Compton parameter GLAST could give constraints
on the magnetization of ejecta
Kobayashi et al. 2007
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Method 3 Polarization measurements
Magnetized jets threaded with a globally
ordered mag. fields which originated a the
central engine, and advected outwards with the
expanding flow. Polarization measurements of
the ejecta emission are very interesting!
Prompt gamma-rays CoburnBoggs2003
controversial Reverse shock emission -- optical
flash Mundell et al. 2007
-- radio flares
GranotTaylor2005 X-ray flares? in the near
future?
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Radio Flares Reverse shock ejecta cools
adiavatically and radiates at lower and lower
freqs at later times. The emission peaks in the
radio after about 1day VLA data Linear
polarization GRB 990123 Plt23 at 1.25days GRB
991216 Plt11 at 1.49days
Plt 9 at 2.68days GRB 020405 Plt11 at 1-2days
GranotTaylor2005
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Early polarization measurements optical

• Polarimeter on our 2m robotic telescope

Liverpool telescope
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GRB 060418

• Afterglow polarization measurement
• 200 sec after the start of prompt gamma-ray
• At the onset of afterglow (12mag)
• Polarization 8 upper limit

Mundell et al. 2007
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IR(REM), XRT
IR smooth rise fading away with a unbroken
power-law the lack of color change steep
rise it is not due to the passage of the typical
frequency of the forward shock emission Onset
of the afterglow should be below
optical at that time 50 photons come from
RS If RS region has global mag fields, we
expect strong polarization. ruling out the
presense of a large-scle mag. fields in the
emitting region.
Molinari et al. 2006
Optical(LT)
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Ruling out the presence of a large-scale mag.
fields in the emitting region. Ruling out the
presence of a large-scale mag. fields in the
ejecta.
Poynting-flux dominated jets high magnetic
pressure might suppress RS. No shock. No RS
emission. No Polarization (the peak might contain
only FS emission) Our results still allow
Poynting-flux dominated jets Reconnections?
L
t
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• If we detect high polarization in early early
  afterglow
• a large-scale mag. fields in the ejecta?
• How fireball jet structure affects the conclusion?

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Large Polarization

• Magnetic field is ordered.
• b) Random Magnetic field Specific viewing angle
  
• The line-of-sight to GRB runs along the edge
  of a jet cone.

Waxman 2003
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Random Magnetic fields generated by
instabilities The mag fields parallel and
perpendicular to the shock normal could have
significantly different averaged strengths
(MedvedevLoeb1999) Some degree of alignment if
observed edge-on If the slab is observed
edge-on, the radiation is polarized!
Ghisellini Lazzati 1999 Gruzinov1999Sari1999G
ranot2003Nakar2004Fan et al.2008
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If the emitting slab moves with a relativistic
velocity, we have to take into account the
relativistic aberration of photons.
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It the line-of-sight to GRB runs along the edge
of the jet cone, we might observe large
polarization. but it is rather rare to see a GRB
from the preferable angle by chance.
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Structured GRB Ejecta
Meszaros et al. 1999 ZhangMeszaros2002Rossi et
al. 2002)

• Initial angular distribution of Lorentz factor
  is not determined from late afterglow obs.
• Deceleration radius is a function of viewing
  angle.
• Deceleration the onset of afterglow
• At the onset of Afterglow, the line-of-sight
  runs along the edge of the emitting jet cone,
  polarized emission is expected!

Jet decelerates around the center first.
GranotKumar2003
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a large-scale mag field in ejecta
L
optical
X-ray
t
t
Structured jets with random mag fields
optical
x-ray
t
t
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Summary

• RS/FS modeling for a few early optical afterglows
• magnetization in RS region is higher
• still baryonic ejecta
• no optical flashes detected in most cases
• ejecta magnetization changes from burst to
  burst???
• Optical Flash/Radio Flares
• no presence of global magnetic fields in the
  emission region
• still Poynting-flux dominated possible
• Need more polarization measurements
  (opt/X-ray/Gamma)
• events with a clear optical flash peak
• Opt/X-ray Polarization measurements might
  constrain the structure of ejecta